Intake manifolds for multi-cylinder engines comprise a branch with a plurality of pipe elements for supplying air, originally guided from an air cleaner to a collector, to each cylinder of the engine. It is known that if the length of this branch is increased, the intake efficiency is improved due to the inertia of the air.
However, in the case of a V-type automobile engine having two banks of cylinders, installation space is limited, so in manifolds having a long branch, an arrangement is adopted wherein the manifold for example forms an overhang above one of the banks and a lateral surface of the cylinder head in each bank and a collector above the bank are connected by a curved branch.
Long manifolds having an overhang tend to vibrate easily, so special care must be paid to the structure that is used to support them.
In Jikkai Sho 62-111966 published by the Japanese Patent Office in 1987, for example, an intake manifold for a V-shaped engine is split into a lower branch having an inverted V-shape so as to connect intake ports of two banks, and an upper branch with an overhang connected to a collector above one bank. Flanges on the ends of each of these branches are fixed by means of bolts.
In this case, on the side on which the upper branch overhangs, the area above the flanges is covered by the plurality of pipe elements forming the branch, so lack of space makes it difficult to pass bolts through the flanges. Some of the pipe elements are therefore bent laterally, in order to make gaps between adjacent pipe elements, and the bolts are passed through the flanges via these gaps. Hence, the bolts can be disposed at effectively equidistant intervals on the flanges, and the lower branch can be rigidly joined to the upper branch.
Instead of rigidly joining the lower branch and upper branch in this way, Jikko Sho 61-23648 published by the Japanese Patent Office in 1986 discloses a structure wherein an intake manifold is elastically supported by a shock-absorbing gasket and a rubber shock absorber.
According to this construction, a lower branch of the intake manifold connected to the cylinder head and an upper branch extending above the cylinder head are bolted together after interposing a shock-absorbing gasket. The other end of the upper branch is supported by a bracket provided in the cylinder head via a rubber shock absorber.
By elastically supporting the intake manifold instead of increasing the rigidity of the support, the sealtightness of connecting parts can be maintained using a relatively small number of bolts. Furthermore, the elastic parts prevent cracks in the manifold or damage of associated parts caused by engine vibration, and also reduce the noise of vibration.
Hence, if it is desired to make a rigid connection between the lower branch and upper branch, a large number of bolts are required to maintain rigidity, the rigidity of the branches themselves must be increased, and as the number of bosses through which the bolts pass must also be increased, the casting of the branches is rendered more difficult and their weight increases.
In the latter construction, on the other hand, a plurality of shock-absorbing supports are required, and costs are higher due to the increased number of parts. The provision of a plurality of shock-absorbing supports moreover imposes design limitations.